For conversion of images that have been rasterized at a high resolution into low-resolution images, a rendering device that improves image quality of vector graphics by performing anti-aliasing via filtering has conventionally been proposed.
Such proposed rendering devices include, for example, the rendering device recited in Patent Literature 1. From (i) stencil data that represents a first image G1 that includes a 2D shape SH1 generated from vector data and stored in a stencil data storage unit, and (ii) coverage data indicating alpha values of pixels located near the edge of the 2D shape SH1, as shown in FIG. 29A, the rendering device recited in Patent Literature 1 generates raster data representing a second image G2, which includes a 2D shape SH2 and is a higher resolution than the first image G1, as shown in FIG. 29B, and stores the second image G2 in a raster data storage unit. Next, after synthesizing pieces of raster data that represent a plurality of 2D shapes SH1 and SH2 stored in the raster data storage unit, the rendering device converts the data obtained by combination into data representing a third image G3, which includes the 2D shapes SH1 and SH2 and is a lower resolution than the second image G2, as shown in FIG. 29C, and displays the third image G3 on a display unit such as a display.
However, in the rendering device recited in Patent Literature 1, it is difficult to prevent an increase in required memory storage capacity since it is necessary to store raster data representing a second image G2, which includes high-resolution 2D shapes SH1 and SH2, in the raster data storage unit, causing the storage capacity of the memory composing the raster data storage unit to grow large. It is also difficult to prevent an increase in the number of memories, since it is also necessary to provide a separate coverage data storage unit to store coverage data.
To address these problems, another rending device has been proposed, the rendering device being provided with a stencil buffer storing stencil data corresponding to the first image G1, a frame buffer storing raster data (frame data) corresponding to the second image G2, a display that displays the third image G3, and a processing device that achieves functions such as generating stencil data and frame data by executing appropriate programs. As shown in FIGS. 30A-30C, this rendering device sets the resolution of the second image G2 to be the same as the third image G3 and sets the resolution of the first image G1 to be higher than the resolution of the second image G2.
In this rendering device, when the 2D shape SH1 is rendered on the second image G2, the image quality of vector graphics is improved by setting color values so as to blur the color of pixels at the edge of the 2D shape SH1 based on an anti-alias pattern. For example, if the anti-alias pattern is set in the first image G1 to four pixels high by four pixels wide, one pixel of frame data in the second image G2 is generated from stencil data in the first image G1 that is four pixels high by four pixels wide.
This rendering device includes a processing device that has an internal processor and main storage device, and when stencil data or frame data is generated, data is transferred between the processing device and the stencil buffer or frame buffer. However, in order to improve processing speed, data transfer between the processing device and the stencil buffer or frame buffer is performed via a method such as burst transfer, which transfers a predetermined data amount (hereinafter referred to as “memory bandwidth”) for one access request. In the example in FIG. 31A, for each request to access the stencil buffer, the processing device accesses four pixels high by four pixels wide of stencil data for the first image A in the stencil buffer.